1. Field of the Invention
The present invention relates to motor vehicle electrical systems having multiple electrical generators.
2. Description of the Related Art
Some motor vehicles have accessories with particularly high electrical loads. An example of one such motor vehicle is an ambulance. In motor vehicles with high electrical loads, generating the electrical power required can be a challenge.
One possible way to generate the electrical power required is through the use of a very high capacity alternator. However, the larger the power generating capability an alternator has, the larger the physical volume the alternator will occupy. Packaging constraints in many motor vehicles preclude the use of a single, very large alternator.
An alternative way to generate the required electrical power is through the use of two or more smaller alternators. The smaller alternators, although perhaps occupying a greater total physical volume than a single larger alternator, provide greater packaging flexibility. That is, space may be more readily available for two smaller alternators than for one large alternator.
In a particular vehicle system having two alternators, an issue has been identified. Where both alternators generate power while the glow plugs of a diesel engine are commanded ON, the voltage provided to the glow plugs is likely to exceed the voltage threshold at which protection measures for the glow plugs take effect. Such measures include cycling the power to the glow plugs to protect them from damage. Cycling the very large current consumed by the glow plugs (up to 100 amperes) can disrupt the idle control system of the vehicle and cause a fluctuating idle speed. This can be a source of customer dissatisfaction. Turning off one of the alternators when the glow plugs are energized will reduce the likelihood of this condition.
There are also other situations where it would be beneficial to turn off one alternator of a dual alternator system. For example, during wide-open-throttle acceleration, turning off one alternator would temporarily reduce the mechanical load on the engine, allowing greater acceleration capability for the vehicle. Also, during idle, reducing the mechanical load on the engine by turning off one alternator will help control the engine""s idle speed within a precise range. Further, reducing the mechanical load on the engine during crank will help assure that sufficient cranking speed is maintained for enough time to start the engine.
Thus, a multiple-alternator system which allows for selective disabling of one of the alternators will provide system performance advantages over alternative systems.
The present invention provides a motor vehicle charging system comprising a first alternator including a first power output and a discrete alternator enabling input, a second alternator including a second power output and an electronic controller having a switchable output. The switchable output is coupled to the alternator enabling input.
The present invention also provides a method for controlling a motor vehicle charging system having a first alternator, a second alternator and a source of switched system voltage having an ON state and an OFF state. The method comprises providing continual enabling power to the first alternator whenever the source of system voltage is in the ON state. The method also includes providing switchable enabling power to the second alternator whenever the source of system voltage is in the ON state.
The present invention further provides a second method for controlling a motor vehicle charging system having a first alternator, a second alternator and a source of switched system voltage having an ON state and an OFF state, and wherein the motor vehicle includes an engine. The method comprises providing continual enabling power to the first alternator whenever the engine is running. The method additionally comprises providing switchable enabling power to the second alternator whenever the engine is running.
By allowing for selective enabling of an alternator in a multiple alternator system, the present invention provides operational advantages over alternative system designs.